Pseudokarst in Guintabon volcanic bombs

Photo 1. Road going to Lake Balinsasayao.
San Jose is a municipality of Negros Oriental.
Taken from: http://www.thelonerider.com/2010/
dec/balinsasayao_bike/images/terrain_map.jpg

The term pseudokarst refers to karst-like features that formed wherein solution is not the dominant process, in contrast to true karsts. Among many examples that would not be discussed here, pseudokarst in volcanic landscapes have been described, such as lava tunnels, lava tubes, lava stalactites, lava stalagmites, and rough surfaces above a lava field, where the ceilings of lava tubes have collapsed. 

Here, I would like to make a case of probable pseudokarst features in volcanic bombs found along the footslopes of Guintabon in Negros Oriental, Philippines.

A fine example of a volcanic bomb showing pseudokarst features is shown in the photograph above. Its composition is that of basalt, with numerous andesite xenoliths. This example measures roughly 2 m x 4 m. While it is not the only example found in the area, this example shows the most well-developed pseudokarst features.

Photo 2. Volcanic bomb showing probable pseudokarst features, found along the road halfway to Lake Balinsasayao. Note the flute-like features, much like solution runnels (rinnenkarren) found in karst environments. Scale: Human around 5 feet high.

What is easily seen in this volcanic bomb are fist-wide vertical flutes rimming the circumference of the bomb, much like the solution runnels (rinnenkarren) found intrue karsts. The interior of the runnels are smooth walled, and show no difference in appearance compared to other parts of the  bomb. This likely points that these runnels formed at the same time the volcanic bomb is cooling, that is, when it was still up in the air.

Photo 3. A close up of the volcanic bomb shown in Photo 1. One of the runnels (left side of the picture) appears to be smoothed by overland flow, but overland flow alone is not likely to produce these features, as explained below. A hornblende-phyric andesite xenolith is found in a cavity 10 inches below the compass, and a larger one 10 inches SE of the compass. Vesicles can also be observed. The other white spots are lichen growths. Scale: A compass, roughly 5 inches in diameter. 

The volcanic bomb is shaped akin to that of a gas stove flame, with curved bases and flared tops. I think this shape is most likely a response to the aerodynamic drag such a large bomb creates as it falls from the sky.

But what is clear is that these features are not a result of solution, such as by rain. For a place like Philippines in which rain is ubiquitous, such features, if a result of solution by rain, should be the norm rather than the exception.